Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) comprising a solid polymer electrolyte or ion exchange membrane disposed between two planar electrode diffusion layers or substrates (diffusion media) formed of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. Suitable carbon fiber paper sheet material is available, for example, from Toray Industries, Inc. with grade designations such as TGP090, TGP060 and TGP030 having thicknesses of 0.27 mm, 0.19 mm and 0.10 mm, respectively, and a porosity of approximately 70%. Carbon fiber paper sheet material is also available in other thicknesses and porosities. Typically, the structure of the electrode substrate is substantially uniform, on a macroscopic scale, as it is traversed in plane (parallel to the planar major surfaces of the electrode substrate, i.e. the XY plane of FIG. 1) at any depth.
The MEA contains a layer of electrocatalyst, typically in the form of finely comminuted platinum, at each membrane/electrode substrate interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel stream moves through the porous anode substrate (anode diffusion media) and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate (cathode diffusion media) and is reduced at the cathode electrocatalyst layer to form a reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product.
In electrochemical fuel cells, the MEA is typically interposed between two fluid flowfield plates (anode and cathode plates). The plates act as current collectors, provide support to the MEA, provide means for access of the fuel and oxidant to the anode and cathode surfaces, respectively, and provide for the removal of water formed during operation of the cells. As the oxidant stream travels through the fluid flow channels typically formed in the fluid flowfield plates of the cell, the stream transports water passing through the cathode diffusion media that is produced as the product of the electrochemical reaction. The water is transported either as water vapor or as entrained water droplets. As a result, the portion of the flowfield into which the oxidant stream is introduced and through which the oxidant stream initially flows is drier than the portion of the flowfield through which the oxidant stream flows just prior to being exhausted from the fuel cell. In the latter portion of the oxidant flowfield, the oxidant stream can become saturated with water, in which case two phase flow occurs, that is, the oxidant stream contains water vapor and also has liquid water entrained in the stream.
Wet and dry regions of the flowfield can detrimentally affect fuel cell performance and accelerate the degradation of performance over time. Fuel cell performance is defined as the voltage output from the cell for a given current density; the higher the voltage for a given current density, the better. Control of water transport perpendicular to the plane of the electrodes (the Z direction of FIG. 1) away from the cathode to the oxidant flowfield; that is, movement of water in the direction from the cathode electrocatalyst layer to the oxidant flow channels (the “free stream”), is important to optimizing fuel cell performance. The “free stream” is the fluid stream within the reactant distribution channels.
In addition to the control of water transport, control of oxidant transport in the direction from the oxidant flow channels or free stream to the cathode electrocatalyst layer, is important to optimizing fuel cell performance. The concentration of oxygen at the electrocatalyst layer directly affects, fuel cell performance because oxygen concentration affects the rate of the electrochemical reaction.
The diffusion media serve several functions. One of these functions is the diffusion of reactant gases therethrough for reacting, within the respective catalyst layer. Another is to diffuse reaction products, namely water, away from the catalyst layer. Additionally, the diffusion media must conduct electrons and heat between the catalyst layers and flowfield plates.
The water generated by the electrochemical reaction on the catalytic layer on the cathode side mostly leaves the electrode as a vapor and condenses in the cathode diffusion media. If the condensed water remains around the catalytic layer, the oxygen gas is prevented from reaching the reaction area and cell performance is lowered. Avoiding a continuous water film in reactant gas pathways is critical to maintaining fuel cell performance.
To solve these problems, various countermeasures have been proposed and tried in the prior art, which include the use of a surface layer or layers on the diffusion media. A micro-porous layer (MPL), well known in the art, consisting of carbon or graphite particles mixed with a polymeric binder is the most common surface layer applied to the surfaces of diffusion media. An MPL has, typically, a pore size between 100 nanometers and 500 nanometers whereas diffusion media, typically, have pore sizes between 10 micrometers and 30 micrometers. Thus, the pore size of an MPL is smaller than the pore sizes of the diffusion media on which it is applied. This is one reason, among others, that an MPL provides an effective way to remove product water from the electrode. It also may reduce electrical contact resistance with the adjacent catalyst layer. An MPL may be applied to a surface of a diffusion media by, for example, screen printing, knife coating, and spraying, and is usually tailored, for example, empirically, to provide a surface coating of desired thickness with the given application technique. A commercial example of an MPL is known as Electrode Los Alamos Type produced by DeNora North America, Etek Division. Micro-porous layers are also described in various literature, for example, “Handbook of Fuel Cells—Fundamentals, Technology, and Applications”, edited by Wolf Vielstich, Hubert Gasteiger, Arnold Lamm, Volume 3, “Fuel Cell Technology and Applications”, Chapter 46, copyright 2003 John Wiley and Sons, Ltd.
It is pointed out that if the diffusion media are given a hydrophilic treatment, the cell performance will drop because the hydrophilic treatment would inhibit transfer of excess water to the flowfield for ultimate removal from the fuel cell.
FIG. 1 depicts a typical prior art subsection 100 of a fuel cell employing a first MPL 102 adjacent to an anode catalyst 104 on the surface 106 of an anode diffusion media 108 and a second MPL 110 adjacent to a cathode catalyst 112 on the surface 114 of a cathode diffusion media 116, wherein 118 is the solid polymer electrolyte or ion exchange membrane. The MEA 120, typically, consists of, collectively, elements 104, 118, and 112.
Other methods have also been proposed to remove water from the catalyst layers, especially on the cathode side, such as placing holes in the diffusion media and embossing a pattern onto the diffusion media. Placing holes in an MPL having a uniform thickness perpendicular to the plane of the electrodes (the Z direction of FIG. 1) and a pattern of hydrophobic treatment on hydrophilic diffusion media have also been proposed. U.S. Pat. Nos. 5,840,438, 6,117,579, 6,521,369, and 6,579,639 exemplify these methods.
However, placing holes in an MPL or the diffusion media or embossing the diffusion media compromise the mechanical, electrical and thermal integrity of the diffusion media. Hydrophilic treatments of the diffusion, media would inhibit transfer of excess water to the flowfield for ultimate removal from the cell. In addition, by situating a MPL having a sharp interface with the substrate perpendicular to the plane of the electrodes (the Z direction of FIG. 1), water within the diffusion media can form a film at the MPL-substrate interface, for example surfaces 106 and 114 of FIG. 1. Such a film, particularly if continuous, would significantly reduce transfer of reactant gases and reduce the limiting current.
Accordingly, what is needed in the art is a method providing better management of reactant gases and water within fuel cells.